Earth satellites are finding increasing use as transponders for communications systems. The use of satellites for communication links between cities eliminates the need for land communication cables, which are very costly. In order to provide continuous coverage, a satellite must be in a geosynchronous orbit. Such orbits require that the satellite be at an altitude of about 22,000 miles. Thus, communications by way of a geosynchronous satellite requires transmission over a path length of 22,000 miles to the satellite and transmission from the satellite over a 22,000 mile path length to the receiving Earth station. Effective transmission over such a distance requires relatively high antenna gain. The necessary gain is achievable with antennas of reasonable size and reasonable cost only at microwave frequencies and at frequencies higher than microwave (hereinafter referred to simply as microwave).
The transmission of signal from the satellite to the Earth station requires a power amplifier located in the satellite capable of generating tens or hundreds of watts of microwave power with great reliability. In the past, the microwave power was generated by travelling wave tubes (TWT). Travelling wave tubes were, and continue to be, used for satellite transmitters notwithstanding the reliability problem attributable to the inherent degradation resulting from operation of a heated cathode over a period of time. More recently, solid state power amplifiers (SSPA) have been used at lower microwave frequencies, such as at C-band, instead of travelling wave tubes. The SSPA has no inherent degradation mechanism, so is more reliable than the TWT. A need exists to provide solid state power amplifiers at X-band (around 10 GHz) and at millimeter wave frequencies (as, for example, 30 GHz).
Solid state power amplifiers are implemented by using a large number of relatively low power solid state devices. Each solid state device provides a small portion of the total output power, and power combiners are used to combine the powers from each of the individual solid state devices to generate the desired amount of signal power at microwave or millimeter wave frequencies.
Various types of power combiners are described in the article "Microwave Power Combining Techniques" by Kenneth J. Russell, published in the IEEE Transaction Microwave Theory and Techniques, May 1979. In the Russell article, corporate or tree combiners are described as being useful for combining a small number of devices but as being very inefficient as the number of devices combined increases. Similarly, the chain type of combiner is not useful. Russell also describes resonant and nonresonant N-way combiners. Among the more successful techniques which he described for combining power are the cavity combining technique. However, this technique has limited bandwidth.
One problem associated with SSPA amplifiers for satellite applications is that of heat dissipation. Water cooling is not practical in a space environment, and there is little or no atmosphere to provide convection cooling. As a practical matter, all cooling of the amplifier modules of an SSPA for space use must be accomplished by conduction. It is a complex problem to make a broadband SSPA which is reliable, small, and in which the heat dissipated by the active elements is carried away by conduction.
U.S Pat. No. 4,291,278 issued Dec. 22, 1981, to Quine describes a power amplifier including a feed waveguide, a fin-line array transition from waveguide to microstrip, a plurality of amplifiers each of which is fed from microstrip, a plurality of phase shifters at the output of the amplifiers for compensating phase, and a fin-line array transition from microstrip to waveguide. This structure requires a phase compensator for each amplifier in order to compensate for the different path lengths from the common feed point to each amplifier, and has the additional problem of requiring alignment of the phase compensators. Furthermore, each phase compensator presumably has a different loss and this results in combination of unequal powers. As the number of amplifiers increases from a few to a very large number, the length of the transmission lines to and from the amplifier most remote from the feed point tends to reduce the effectiveness of the structure in combining the power.
A power amplifier is desired which is easy to manufacture and suitable for use at microwave and millimeter wave frequencies, does not require phase correction for differing line lengths, in which each amplifier module may be provided with positive heat sinking, and each module can be accessed for maintenance without substantial disassembly of the structure.